Algodoo

Here you can see two gearwheel "mashines".
The upper will be blocked by a little stone quite at the beginning. After a while, another stone
will appear from above. This stone, smaller than the first, will attack the bigger "mashine".
However, it won't block the "mashine". Instead of that, it will just bounce around and will disappear
from the screen. Then it is all up to the bigger stone. The said one will take his chance, will jump down,
and finally block the bigger "mashine".
Note that you maybe have to set the time speed higher when it comes to the bigger stone attacking the
bigger "mashine" because it maybe takes hours.